Some conventional optical inspection tools locate defects on patterned wafers by scanning the surface of the wafer with a tightly focused laser spot and measuring the amount of light scattered by the illuminated spot on the wafer. Dissimilarities in the scattering intensity between similar locations in adjacent dies are recorded as potential defect sites.
The dynamic range of this optical scattering is typically substantial. Changes in scattering intensity of more than a million to one within a single die are not uncommon. This high dynamic range is intrinsic to the optical configuration of the instrument and the scattering properties of the wafers and defects of interest.
Optical sensing is the process of converting optical signals (photons) into electrical signals (electrons). When the optical signals are dim, and frequencies are high, photomultiplier tubes are typically used. A photomultiplier tube generally includes a photocathode, one or more dynodes, and an anode. Individual photons striking the cathode have a particular probability (e.g., 25%) of dislodging an electron. These photoelectrons are then accelerated towards the first dynode by an electric field. When the electrons strike the dynode they dislodge additional electrons, thus amplifying the signal. These secondary electrons then cascade towards the next dynode where they are again amplified. At the end of the dynode chain, the electrons are collected by the anode which carries them outside of the photomultiplier tube. At this point, the signal is large enough to be easily measured using conventional electronics, such as a transimpedance amplifier, followed by an analog-to-digital converter.
The gain at each dynode is a function of the energy of the incoming electron, which is proportional to the electric potential between that dynode and the previous stage. The total gain of the tube is the product of the gains from all the dynodes. There are continued efforts to improve photomultiplier tube gain control.